DESCRIPTION (Applicant's Description): New evidence for photoreceptor involvement before and after resolution of the active, vascular phase of retinopathy of prematurity (ROP) leads to the main hypothesis of this project: the greater the deficit in photoreceptor function the more severe the acute phase ROP. Infants (enrolled at age 10 wks) and children (enrolled at age 8-12 yrs) categorized by severity of ROP (None, Mild Moderate, Severe) will have electroretinographic (ERG) and psychophysical investigations of photoreceptor function. In all subjects, the parameters of the activation of rod phototransduction, and dark-adapted rod thresholds will be measured in parafoveal and peripheral retina. Rod response parameters will be analyzed for significant variation with ROP category. Retinal mechanisms represented in post receptoral responses, and the relationship of receptoral to post-receptoral functions, will also be analyzed. High refractive errors in ROP frequently develop during the years when photoreceptors mature. In this project about retinal mechanisms, the secondary hypothesis is: the greater the photoreceptor dysfunction, the greater the refractive error. All subjects will have cycloplegic refractions, as well as acuities, measured. To evaluate the developing retinal processes and spherical equivalent for causative relations, a longitudinal study of rod thresholds, acuity and refractive error between age 10 wks and 3 yrs will be conducted. The parameters of these courses, as well as the retinal response parameters obtained in the other studies of retinal function, will be used to test the secondary hypothesis. In the children, other photoreceptor functions including deactivation of rod phototransduction, activation of cone phototransduction, and rod increment threshold functions will be investigated. The parameters obtained will be used in further tests of the main and secondary hypotheses, and for within subject evaluation of relative involvement of rods vs. cones, activation vs. deactivation, and receptoral vs. post-receptoral mechanisms. This project, based on an entirely new perspective of ROP, will lead not only to much more information about retinal and visual function in ROP, but also to new knowledge about fundamental ROP disease processes.